The present invention relates to a riser control device, particularity designed to be used in connection with spool or horizontal production trees in sub-sea oil and gas installations.
The past decade has seen the use of sub-sea production systems become the method of choice for exploiting offshore oil and gas fields. The use of these systems offer significant advantages over traditional platform based methods in both economics and reservoir management terms. A significant change in sub-sea production systems occurred with the introduction of the spool or horizontal production tree. Enabling the use of large bore completions and subsequently multi-lateral wells, the introduction of this equipment has led to a considerable reduction in the number of wells required to fully exploit an offshore field. These systems also reduce capex and opex costs by enabling completion and intervention operations to be conducted via a traditional drilling riser and BOP (blow out preventer) as opposed to the dual skeletal riser normally associated with conventional sub-sea production trees.
Many of the fields developed with horizontal trees are now moving into the 2nd phase of production and consequently the intervention phase, i.e. extensive production logging programs followed by the diagnosed remedial operations such as re-perforating and water shutoff activity, the requirement for and difficulty of these operations is increased by the complexity of reservoirs both developed and planned. The very nature of the wells with long horizontal sections undulating through the producing section require the deployment of intervention tooling on compressively stiff coil tubing. The critical function when deploying equipment of this type in a sub-sea environment is the ability of the sub-sea LRP to cut the intervention string and isolate the well. Current well isolation devices utilized for this service are based upon established techniques utilized in down hole safety valves, with the primary cutting device being a ball valve and the primary sealing device being either a ball or flapper valve.
The use of a ball valve to provide a cutting function is unique to this type of application as cutting operations are normally conducted by BOP's which offer considerable advantages as the cutting efficiency is much greater and debris tolerance is significantly greater thereby providing improved sealing reliability.
A further major influence on intervention policy will be the ability to deploy the intervention system and conduct operations from a lightweight vessel.
In the formative era of the horizontal tree, it was envisaged that intervention operations would be conducted from a drilling rig via a marine riser/BOP and a large bore work over riser and LRP. However, the use of a conventional vessel involves cost implications not only with high opex but also with the increased complexity of mooring in and around production facilities and infrastructure. Many studies have been conducted to establish the economic and operational integrity of conducting interventions from a lightweight semi or mono hull vessel. The size of these vessels preclude the use of a marine riser and BOP stack, requiring the deployment of a sub surface lubricator system similar to that used on conventional tree interventions. Well control during these operations is achieved by a combination of barriers contained within the intervention system and the production tree. This enables full flexibility of well containment and even the complete retrieval of the intervention equipment, with the valves contained with in the vertical bore of the production tree providing well isolation.
However, when conducting similar operations on a horizontal tree with no vertical isolation capability (both tubing hanger and tree cap plugs removed to allow intervention string access), the only vertical isolation available is contained within the intervention system itself. Under normal circumstances this meets with accepted barrier philosophy but does preclude the ability to remove the intervention equipment or deploy a BOP for well kill or fishing operations. Several different concepts to improve the integrity of horizontal tree lightweight intervention operations that have been produced all allow the deployment of a drilling BOP during intervention operations, such as the use of a connector, shear ram and connector spool (the shear ram providing well isolation during intervention system running and pulling) or the deployment of a connector and spool with an integral internal valve which can be hydraulically closed enabling the intervention system to be retrieved and the BOP stack to be run. Both systems, however, add considerable weight to the intervention system, and consequently require a much larger vessel than those normally associated with lightweight intervention techniques. A further disadvantage is that the bending moment induced at the production tree and wellhead is substantially increased by the weight and length of the spool and additional connector, thereby precluding the use of this system in all but benign environments.